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What Size Wire for a 45 Amp Breaker?

A 45 A breaker needs 6 AWG copper in NM-B cable (“Romex”), or 8 AWG copper in conduit with 75°C terminations. NEC 334.80 caps NM cable at the 60°C column, which is why the two differ: most homes are cable, so 6 AWG is the safe default.

Minimum wire size for 45 amps

ApplicationCopperAluminum
NM-B cable / “Romex” (334.80, 60°C)6 AWGnot typical
Conduit / SER, 75°C terminations (310.16)8 AWG6 AWG
Worth knowing: 8 AWG is the 75°C minimum in conduit, but 8 AWG NM-B is only 40 A under NEC 334.80: cable runs step up to 6 AWG.

The NM-B row applies the 60°C column per NEC 334.80; the conduit row assumes 75°C-rated terminations (NEC 110.14(C)). Both assume no more than three current-carrying conductors in a raceway at 30°C ambient. Derating, continuous loads, and local amendments can change the answer: confirm with a licensed electrician.

Ground wire for 45 amps

GroundCopperAluminum
Equipment ground (bare / green) (Table 250.122)10 AWG8 AWG

The equipment ground is sized by the breaker, not the load: a 45A breaker falls in Table 250.122's 60 A row. Upsize the hots for voltage drop and the ground must grow in proportion (NEC 250.122(B)). Every size, with a worked upsizing example, is in the ground wire size chart.

How far can you run it? (voltage drop)

Keeping voltage drop under the recommended 3% at a 36 A load (80% of the breaker), 6 AWG copper is good for roughly:

System voltageMax one-way run (6 AWG Cu)
120 V≈ 101 ft
240 V≈ 203 ft

Past these distances, step up one gauge. Run your exact length and load through the voltage drop calculator.

What runs on 45 amps

  • Heat pump with strip heat
  • Large AC condensers

Parts for a 45 A circuit

Before touching a 45 A circuit, confirm it is dead with a voltage pen; a clamp meter reads the real load on one of its conductors.

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Frequently asked questions

It depends on the wiring method. In NM-B cable ("Romex"), the usual residential case, NEC 334.80 requires the 60°C column, so you need 6 AWG copper. In conduit with 75°C terminations, 8 AWG copper is the Table 310.16 minimum. In aluminum, use 6 AWG. When the method isn't settled, run 6 AWG; it is compliant either way.

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